This invention relates to high-pressure capable end fittings on tubing and, more particularly, to a sleeve, a tool assembly, and a method of using the sleeve and tool assembly to form axially-swaged, male end fitting assemblies for use with cone female fittings.
Aerospace and high performance commercial hydraulic system applications commonly require high-pressure fittings to connect lengths of tubing. Important attributes of superior fittings include pressure capability, size and weight, and tensile, torsional and bending strength. The time, cost and likelihood of defects occurring in the making of a fully qualified and inspected fitting are important aspects to consider in selecting which fitting is appropriate for a given application. The 24.degree. cone flareless female fitting has been one of the most widely accepted hydraulic fitting types used for aerospace and high performance commercial hydraulic system applications. Numerous variations of the male end fitting assemblies used with this well-accepted female fitting are presently produced. However, all such variations suffer deficiencies in one or more of the above attributes and selection aspects.
The most commonly used male end fitting assembly for a 24.degree. cone flareless female fitting (see FIG. 1) comprises essentially a nut 101 and a shouldered sleeve 102 to be placed over the end of a tube 103. The sleeve includes a shoulder 104 and a cylindrical nose 105 oriented to face a female end fitting 106 when the fitting is assembled. The shoulder has corners that are not rounded, being machined only enough to remove burrs and break sharp edges. Thus, on a half-inch fitting, the corners' radii are likely to be on the order of 0.002 to 0.006 inches. The female fitting has a threaded outer surface 107, and a conical inner surface 108 that receives the tubing and sleeve of the male end fitting assembly. When the fitting is first assembled, the nut is screwed onto the threaded outer surface, forcing the nose end of the sleeve into a gap existing between the conical surface and the tube. As the nut is advanced, the nose end of the sleeve deforms radially into the tube, forming a permanent "preset" deformation 109. The tube and sleeve become a unit, and thus the fitting may then be disassembled and reassembled without disturbing the preset.
Although the 24.degree. cone flareless fitting is an effective fitting, the male end fitting assemblies used with it suffer from limited strength, limited flexural endurance, and a susceptibility to over-torquing. In particular, the strength and endurance are limited by the form of connection between the sleeve 102 and the tube 103. The connection is formed by a localized circular "dig-in point" 110 at one axial location around the tube where the nose 105 end of the sleeve digs into the tube during presetting. This dig-in point provides a limited contact area, leading to weakness in tensile and torsional strength. At the same time, the dig-in point is a location of high stress concentration, leading to reduced bending strength and poor flexural endurance. No significant support, to improve bending strength, comes from the length of the sleeve because the necessary diametral gap between the tube and the sleeve does not allow the sleeve to pick up any significant load before the preset stress concentration point (the dig-in point) is over-stressed.
During tightening of the fitting nut 101, the fitting might easily be over-torqued. Over-torquing causes excessive digging in by the end of the nose 105, locally constricting fluid flow through the tube 103. Eventually, over-torquing causes the sleeve's shoulder 104 to bottom out against an end 111 of the cone flareless female fitting, diminishing the ability of the fitting to subsequently seal on later reassembly. While the adverse effects of over torquing may be overcome on initial factory assembly by using presetting tools, they frequently reappear during field servicing when wrenches are used to manually tighten and retighten the nut.
Numerous attempts have been made to improve the performance of the male end fitting assembly of the 24.degree. cone flareless fitting. For example, the National Aerospace Standard NAS 1760 male end fitting, shown in FIG. 2, represents a proposed alternative. It provides a one-piece end with a cylindrical nose 112 with a spherically curved tip 113. The nose is shaped to simulate the nose of a male end fitting assembly that has already been preset, and thus includes a cylindrical nose extension 114 to simulate a tube extending out of a sleeve. FIG. 2 depicts a variation 115 of the NAS 1760 attached end-to-end with a tube 116, e.g., by welding, leaving the nose free for attachment to a 24.degree. cone flareless female fitting. Welding, however, is a labor intensive operation requiring exceptional cleanliness, and extensive inspection, generally including the use of x-ray inspection. Thus, welding a NAS 1760 end to a tube is a costly and time-consuming option.
Internal swaging has also been used to attach a sleeve to a tube (see FIG. 3). The tube 117 is inserted into a sleeve 118 designed for internal swaging. The sleeve includes a cylindrical nose 119 with a spherically curved tip 120. The sleeve has an inner surface 121 including axially spaced, circular teeth 122 to seal against the tube when the tube is expanded outward during internal swaging. An internal swaging mechanism is used to expand the tube into gaps formed between the teeth of the internally swaged sleeve. While this provides a male end fitting with strength that is superior to that of the preset male end fitting assembly, the internally swaged fitting is both heavier and longer. The tooling required to perform internal swaging is costly. Furthermore, that tooling is complex and subject to deterioration and failure, leading to defects in the swaged area that are not detectable by simple inspection methods.
Another known approach for securing a sleeve to a tube, shown in FIG. 4, is to use a heat-shrinkable alloy. The sleeve 123 includes a shoulder 124, a cylindrical nose 125 with a spherically curved tip 126, and an inner surface 127 with a plurality of axially spaced circular teeth 128 around the inside of the sleeve. The sleeve is made to its "shrunk" size and then is expanded by a mandrel while immersed in liquid nitrogen. It retains its expanded size so long as it remains in the liquid nitrogen. To join the sleeve to the tube, the expanded sleeve is removed from the liquid nitrogen and placed upon the tubing, where it returns to room temperature and approaches its original size. Because the tube blocks the sleeve from shrinking to its original size, the sleeve remains elastically partially expanded, with its teeth dug into the tube. Thus, a preset male end fitting assembly with a NAS 1760 axially-curved nose is simulated. The torsional strength of the heat-shrinkable male end fitting assembly, however, is no better than that of the "preset" sleeve. The use of liquid nitrogen involves high expense and places time constraints on the attachment operation, which must be done within the few seconds that the sleeve remains expanded after removal from the liquid nitrogen. The cost and difficulty are further driven up by the need to chill and temperature stabilize the tubing end prior to attachment. Also, the cost of heat-shrinkable material is significantly higher than that of standard sleeve materials.
Another known simulation of a preset male end fitting assembly, depicted in FIG. 5, includes two partial sleeves that are axially squeezed between a 24.degree. cone 129 and a nut 130 (or a tool providing the equivalent surfaces). The first partial sleeve 131 includes a surface 132 conforming to the 24.degree. cone, and a second surface 133 at 45.degree.. The second partial sleeve 134 includes a conforming 45.degree. surface 135, and a surface 136 conforming to the nut. Upon squeezing, the first partial sleeve is radially compressed by being pressed into the 24.degree. cone, while the second partial sleeve is radially compressed by being pressed into the 45.degree. surface of the first partial sleeve. The resulting male end fitting assembly has one ring shaped indentation formed within each partial sleeve, and discontinuity of indentation at either end of each partial sleeve.
This simulated male end fitting assembly is difficult to assemble. Twice as many parts must be kept in stock, and caution must be used that the parts are used in the proper respective positions. Use of a nut to form the assembly is limited to very soft tubing because the male end fitting assembly requires a high squeezing force. This high level is required by the steep incline of the 45.degree. angled surfaces 133 & 135, and by the fact that the second partial sleeve 134 pushes radially outward on the first partial sleeve 131 during the squeezing. This high level increases exponentially as tube hardness goes up.
Accordingly, there has existed a definite need for a male end fitting assembly demonstrating high-pressure capability, low size and weight, high tensile strength, torsional strength, and flexural endurance. The installation time, the fabrication cost and the likelihood of over-torquing for the fully qualified and inspected fitting should be minimized. The present invention satisfies these and other needs, and provides further related advantages.